A flexible thin card including an electric battery element is known from Japanese Patent Application JP 60-7056, no. 58-114925 of Jun. 25, 1983, published on Jan. 14, 1985 (Tomoyuki Aoki). This card comprises first and second rectangular, flat and flexible cover foils below and above a flat battery. These foils are positioned face to face and sealed to each other by means of a joint consisting of a sealing material provided in between the foils and at their edges. The battery is arranged in the cavity created by this sealing joint and comprises first and second electrodes in the form of conducting foils, one of which is lengthened and constitutes an external contact lug between the sealing joint and the first cover foil at an extremity of the rectangle, and the other is lengthened and constitutes another external contact lug between the sealing joint and the second cover foil. The sealing operation by means of the joint is realized by gluing or fusion, using ultraviolet radiation, or in a high-frequency electric field eliminating alien particles which might otherwise render the sealing non-impervious.